1. Field of the Invention
The invention relates to an audio transmission circuit for a telephone set which is fed with line direct voltage via the subscriber line, comprising a high-output impedance microphone pre-amplifier and a transmit amplifier which is in the form of a line voltage stabiliser and includes an input stage constituted by a fed-back operational amplifier and includes an output stage which is formed by an output transistor and an associated emitter resistor, which output stage is arranged in parallel with the subscriber line, a signal input of this operational amplifier being connected to the output of the microphone pre-amplifier, and the transmit amplifier further comprising a reference voltage source for producing a reference voltage for line voltage stabilisation.
2. Description of the Related Art
Such an audio transmission circuit is disclosed in the article "A programmable Speech Circuit Suitable for Telephone Transducers", IEEE Journal of Solid-State Circuits, Vol. Sc-17, No. 6, December 1982, pages 1149-1157.
In operation, a telephone set connected to a subscriber line of a public telephone network receives from the telephone exchange via this subscriber line a line voltage which comprises the line d.c. voltage and the received a.c. signal voltage. During transmission, the line d.c. voltage is modulated with the signal to be transmitted by the telephone set.
The administrations prescribe a maximum direct voltage across the set, which maximum direct voltage further depends on the line current. A typical value for the maximum line direct voltage is, for example, 5.7 V at a 10 mA line current. In the telephone set the line direct voltage is set by a voltage stabiliser which may form part of the transmit amplifier.
The audio transmission circuit described in said IEEE article comprises a transmit amplifier which not only stabilises the line d.c. voltage but is also included in a Wheatstone-bridge for producing the anti-sidetone effect and with which in addition the desired line terminating impedance can be actively realized. The properties of the audio transmission circuit as regards the above-mentioned functions are determined by passive components, which also determine the gain of transmit amplifier. Consequently, it is not easy to increase this gain in a simple way without adversely affecting the other properties of the audio transmission circuit.